There is, of course, the old display bug to consider. But that aside:
If you are trying to sell something, and it posts a trade higher than where you were offering, that simply means at the time someone bid that, someone else was offering it cheaper than where you were. Probably working as intended. You can either wait and hope, or you can bite the bullet and repost.
If you are trying to buy something, and it posts a trade lower than where you were bidding, something is wrong. I have seen this happen a number of times, which may have been the display bug, but it leads me to believe that sometimes the bidding system doesn't work as intended.